The present invention relates to a lighting unit and a display having the same.
Displays are media used to visually convey information to humans and play important roles today for humans and the highly sophisticated information society. Displays are classified largely into light emitting type displays, such as CRTs (cathode ray tubes) and PDPs (plasma display panels), and non-light emitting type displays, such as LCDs (liquid crystal displays), ECDs (electrochromic displays) and EPIDs (electrophoretic image displays).
The non-light emitting type displays form an image by adjusting the quantity of light transmission (or light reflection). Of this type of displays, the liquid crystal display in particular has achieved remarkable performance improvements in recent years and has been adopted increasingly as a display from cell phones to personal computers to large-screen television sets.
The liquid crystal display generally comprises a liquid crystal display panel that forms an image by controlling the amount of light passing therethrough or being reflected thereby, and a lighting unit disposed at the back of the liquid crystal panel to throw light onto the panel.
The lighting unit has some lighting schemes—an edge lit scheme (light guide plate scheme), a direct backlight scheme (reflection scheme) and a flat light source scheme. Especially to realize a thin lighting unit, the edge lighting scheme is employed. The edge lighting scheme has a light source arranged at the edge portion of a light guide plate and, as a major light source, a cold cathode fluorescent lamp is conventionally used. In recent years, however, light emitting diodes (LEDs) are being used as a light source of the lighting unit in small liquid crystal displays such as cell phones. Further, because of its advantages of not requiring an inverter necessary for the conventional cold cathode fluorescent lamps, and of being able to reduce environmental burdens, the LEDs as a light source free of mercury have come to be used increasingly in large-screen liquid crystal displays.
In the self-light emitting type displays such as PDPs, when an image is displayed, particular pixels are selectively driven to produce required volumes of light according to image signals. So, displaying black or dark image is achieved by stopping the emission of light or reducing the amount of light produced in the associated pixels, which contributes to a reduction in power consumption. Since in displaying black the associated pixels do not emit light, the contrast ratio in a dark room can be set to higher than several tens of thousands.
In the non-light emitting displays such as general liquid crystal displays, on the contrary, the backlight is made to emit light at a constant brightness level regardless of image signals. So, the backlight normally has its brightness matched to the conditions that produce the maximum luminance on the screen and maintains the same brightness even when black or dark images are displayed, resulting in a wasteful consumption of power that does not contribute to the image being displayed. Further, when black is displayed, the light leaking from the backlight prevents the image from becoming sufficiently dark, so that the contrast ratio in the dark room is smaller than that of the self-light emitting type displays such as PDPs.
There have been proposed liquid crystal displays that reduce power consumption and improve image quality by controlling the brightness (or luminance) of the backlight.
JP-A-2005-258403, for example, discloses a technology that involves driving each of a plurality of divided block areas of a backlight, and controlling the brightness of each block area of the backlight according to an image signal, thereby reducing power consumption and improving image quality.
A variety of techniques have been proposed which control the luminance in each of a plurality of block areas of a backlight. For example, JP-B-4,262,368 discloses a lighting unit (backlight) which comprises a plurality of light guide plates each having a pair of opposing ends with different thicknesses and a plurality of light sources arranged close to the thicker of the pair of ends, different in thickness, of the light guide plates. JP-B-4,023,079 discloses a lighting unit which comprises a plurality of parallelly arranged linear light sources and a light guide plate having a plurality of grooves in which to accommodate these light sources, with the light guide plate formed with an inclined surface that forms a curved line along which the light guide plate, when viewed in cross section, decreases in thickness away from the light source accommodating portions. As a further document describing a prior art related with this invention, JP-B-01-037801 is cited.